An oral ulcer, mouth ulcer, or, in American English, canker sore is an open sore inside the mouth caused by a break in the mucous membrane or the epithelium on the lips or surrounding the mouth. The types of oral ulcers are diverse, with a multitude of associated causes including: physical or chemical trauma, infection from microorganisms, medical conditions or medications, cancerous and nonspecific processes. Once formed, the ulcer may be maintained by inflammation and/or secondary infection. Two common oral ulcer types are aphthous ulcers and cold sores or fever blisters. Cold sores around the lip are caused by the herpes simplex virus.
Lichen planus is a chronic mucocutaneous disease that affects the skin and the oral mucosa, and presents itself in the form of papules, lesions or rashes. Lichen planus doesn’t involve lichens; the name refers to the appearance of affected skin
Acanthosis nigricans is a brown to black, poorly defined, velvety hyperpigmentation of the skin. It is usually found in body folds, such as the posterior and lateral folds of the neck, the axilla, groin, umbilicus, forehead, and other areas.